Air Gap Servo for Optical Recording

Abstract

A device reads and / or records marks in a track on a record carrier via near field optical recording. The device has a head including a lens to be positioned at a near field distance from a surface of the record carrier. An air gap controller is for controlling an air gap between the lens and the surface, and has an approach mode for bringing the lens from a remote distance in the far field (72) to the near field distance. Thereto the controller provides an increasing periodical excitation signal (73) for generating a sequence of approach instants (77) at which the lens approaches the surface. At the approach instants the lens has substantially zero velocity (76). The sequence of approach instants brings the lens subsequently closer to the surface. When the lens enters in the near field range (71) at one of the approach instants (77), the air gap controller is switched to closed loop mode.

Description

FIELD OF THE INVENTION[0001]The invention relates to a device for near field optical recording, information being represented by marks in a track on a record carrier, the device comprising a head including a lens to be positioned by a lens actuator at a near field distance from a surface of the record carrier for generating a scanning spot on the track.[0002]The invention further relates to a pull-in method for bringing a lens from a remote distance to a near field distance from a surface of a record carrier for use in near field optical recording, information being represented by marks in a track on the record carrier to be scanned via a head including the lens.BACKGROUND OF THE INVENTION[0003]An air gap servo for near-field optical recording is known from the document “Gap Servo System for a Biaxial Device Using an Optical Gap Signal in a Near Field Readout System,” by T. Ishimoto, K. Saito, T. Kondo, A. Nakaoki and M. Yamamoto, published at ISOM / ODS 2002 (further referred to as d...

AI Technical Summary

Benefits of technology

[0005]Therefore it is an object of the invention to provide an air gap servo system and pull-in method for optical recording that is fast and has a low risk for a head-disc crash.

[0008]Due to the substantially zero velocity of the lens at the sequence of approach instants the risk of a head disc crash is low, as substantially no overshoot will occur. Due to the periodical nature the movement of the lens is controllable in a fast way. Hence the measures have the effect of speedily bringing the lens to the target near field distance. This has the advantage that the start-up time before a user can start reading or recording on the record carrier is decreased.

[0009]The invention is also based on the following recognition. The inventors have seen that the problem of overshoot can be substantially eliminated by generating the approach instants at which the velocity is substantially zero. This is achieved by the increasing periodical excitation. Surprisingly the total time for the pull-in procedure is reduced when compared to a ramp based open-loop approach operation. In particular a sinusoidal excitation for the approach control guarantees a substantially zero velocity when the air-gap servo system starts its closed-loop operation at the switching instant.

[0010]In an embodiment of the device the air gap controller comprises a reference generator for, in a hand-over mode, providing a reference near field distance changing from a first target near field distance to a second, lower target near field distance via a transfer function. Furthermore, the reference generator is for providing reference values to a controller unit based on a two degree of freedom control technique in said hand-over mode. Advantageously, the two degree-of-freedom control technique for the hand-over control relaxes the design constraints between the overshoot and the settling time.

Problems solved by technology

Hence, during the pull-in procedure overshoot may pose a severe problem, eventually resulting in a head-disc crash if the head comes into contact with the disc surface.

However, the air gap error signal is only available in a near field region of approximately 50 nm.

Due to the late appearance of the air gap error signal and in view of the problem of overshoot the maximum ramp speed is limited, and the pull-in procedure is slow.

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